The present invention relates to feedback control loops of the type wherein the loop error signal is measured. More particularly, the invention relates to increasing the effective bandwidth of the phase error signal in a phase lock loop.
It is known in the prior art to utilize the phase error signal in a phase lock loop as a measure of phase jitter appearing on an input signal. As examples of this technique, reference is made to U.S. Pat. Nos. 3,546,588 (to Campbell, Jr.) and 3,711,773 (to Hekimian et al). The phase error or measurement signal is derived from the loop phase detector and provides measure of phase modulation of the loop input signal over a band of phase modulation frequencies determined by the loop bandwidth. This loop bandwidth is normally designed as a compromise between optimum acquisition to achieve phase lock and optimum tracking to maintain phase lock. Specifically, acquisition is most optimally effected with a wide loop bandwidth wherein the loop can virtually instantaneously lock onto the signal; tracking, once phase lock is achieved, is most efficiently performed in a narrow band loop. Hence, there is a need for compromise in selecting loop bandwidth.
Quite often it is desirable to be able to measure phase jitter at phase modulation frequencies below the low frequency cut-off of the loop passband. One approach to doing this would be to re-design the loop to include the lower frequencies in the loop passband; however, this destroys the compromise referred to above and adversely affects the reliability of the loop operation in the tracking mode. Other possibilities exist, but they involve use of complex circuits and attendent cost increase.
It is therefore a primary object of the present invention to provide a simple and inexpensive technique for measuring phase jitter in a frequency band extending below the low frequency cut-off of a phase lock loop bandwidth without affecting loop operation or design.
It is another object of the present invention to provide a circuit which permits phase jitter measurement to be extended to phase modulation frequencies residing outside the bandwidth of a phase lock loop from which the measurement signal is derived.
In a broader context, it is an object of the present invention to provide a technique wherein the bandwidth of the error signal in a feedback loop can be expanded to frequencies below the low frequency cut off of the loop bandwidth without affecting loop operation.